The purpose of this study is to measure the benefit of adding abemaciclib to chemotherapy (irinotecan and temozolomide) for Ewing's sarcoma that has come back or did not respond to treatment. This trial is part of the CAMPFIRE master protocol, which is a platform to speed development of new treatments for children and young adults with cancer. Your participation in this trial could last 11 months or longer, depending on how you and your tumor respond.